These types of apparatus produce a panoramic image of the dental arches of a patient, exposing the skull of a patient to X-rays. Panoramic radiography (also known as orthopantomography) produces a radiographic image of a curved plane, also known as Welander curve, approximating patient jaws, with blurring of the anatomical structures laying outside a narrow layer around the predesigned curved plane. The set of movements that mechanical parts must perform to achieve this result is called trajectory. This technology has been known since the 1950s. For the first 30-40 years (1950-1990), a film used to be exposed to X-rays. Today these types of apparatus use digital sensors, converting the X-ray impinging on the X-ray detector into an electric signal, which, suitably processed, forms a digital image.
The X-ray bundle undergoes attenuation while passing through different tissues of the patient; this attenuation determines the degree of exposure of a radiographic image. Different tissues attenuate X-rays in different ways according to their density, and this allows distinguishing a tissue from another. The correct exposure is obtained by balancing the intensity of X-ray bundle, so that the tissues of clinical interest become well visible and detailed. With an excessive exposure, tissues of low-medium density are crossed by X-rays without an attenuation that can detected (overexposure), while an exposure that is too low entails an excessive X-ray attenuation by more dense tissues, so that they cannot be distinguished from the neighboring tissues (underexposure).
In case of overexposure or underexposure, a re-take may be necessary. One must always keep into account that X-rays can lead to patient biological damage, as X-rays are ionizing radiations which can damage cell DNA. Therefore, the need to administer the lowest X-ray dose for the individual patient under examination is apparent.
Typically, a panoramic apparatus comprises an X-ray source and detector fixed to the ends of a rigid support, which turns around the patient. It also comprises a patient positioning and immobilizing device, which holds the patient in position during the acquisition.
Technical parameters that influence the intensity of the X-rays emitted from the source are:
X-ray tube power (kV),
X-ray tube current (mA),
duration of exposure (sec), and
film speed.
Originally, in film analog apparatuses, this last parameter used to determine how long a portion of the film was exposed to radiation, and film direction. This parameter in many of today's digital apparatuses was maintained as term, and corresponds to the speed and reading direction of X-ray sensor; elsewhere it is replaced by a parameter called clock TDI. In both cases, its effect on the quality of image exposure is similar.
The exposure for a patient can be adjusted by a professional operator, who can evaluate in advance the right X-ray dose needed for a good radiographic image, on the basis of his/her professional competence. Many manufacturers propose to operator pre-defined settings of these parameters, suitably set according to patient's gender and size, e.g. distinguishing among child, woman, man, and small, medium large size; such a system is described e.g. in U.S. Pat. No. 4,618,974 (Siemens). Anyway, the selection of these settings is always left to human operator.
Over the years, automatic adjusting devices were developed, which set the exposure according to a suitably chosen criterion (best contrast, lowest dose, etc.). These devices are known as AEC (Automatic Exposure Control) devices. The solutions nowadays available on the market can be subdivided into two categories:
parameters adjustment according to a pre-acquisition exposure (also known as scout);
continuous parameters adjustment during the acquisition (also known as run-time AEC).
A first way of the known art to solve the present technical problem consists in devices based on measuring, through dosimeters or similar devices, X-ray attenuation during scout acquisition. These detectors can be placed in one or more areas on the detector side (wither a sensor or a film), and can be fixed, mobile or removable. Such technique is described e.g. in patents U.S. Pat. No. 4,813,060 (Siemens), U.S. Pat. No. 5,425,065 (Instrumentarium) and EP 0574368 (Orion Yhtymae Oy). The detectors provide a direct measurement of X-ray attenuation through patient's tissues.
The disadvantages of this kind of solution are due to the need of specific mechanisms for dosimeter reading, their maintenance, the managing of their volume and the optimization of a work flow, wherein the attenuation measuring step and the following exposure to obtain images need supports, interfaces and data processing different from each other. When digital detectors replaced films, digital detectors replaced the use of specific dose detectors, since digital detectors can be used both for attenuation measuring during pre-acquisition and for image formation during the acquisition. The use of the same type of detector removes the need of correlating different signals (provided from a dosimeter and a digital detector) to exposure, preventing the need of cross-calibrations and cross-checks between the two systems.
Many of these mechanisms, though, do not take into account patient's structure and anatomical peculiarities, or the human operator must choose suitable exposure parameters according to his/her professional experience and competence, looking at scout image. In this case the time needed for the acquisition becomes longer, and a wide scout or more than one scout is required to detect the anatomical portion of interest. Moreover, the use of a scout requires the administration of a further (small) X-ray dose to patient.
If scout image is too small, or parameter computing algorithm is not specific for the acquired anatomic portion, or operator's intervention is not required in choosing parameters, the automatic setting of emission can become very sensitive to patient's positioning or peculiar bone structure.
A second way of known art for solving the present technical problem is a continuous time adjustment of exposure during the acquisition; the most popular methods require complex feed-back control mechanisms of X-ray tube. Such a technique is described e.g. in patent U.S. Pat. No. 4,333,012 (Morita). According to this solution, a control system must be designed, which, during the acquisition, reads at time t the content of detector, processes it to get suitable emission parameters and adjusts the emission at time t+1 according to them. This entails designing X-ray tubes modulating their emission in very short time, with a very stable answer to input parameters and very short transient state, and a very fast system of detector reading and analysis.
In addition to technical complexity and high cost of this solution, one of its main disadvantages is that in situations of peculiar irregularity of patient's anatomy, or in the presence of metallic prostheses, or of a material much denser than adjoining tissues, the value of parameters computed at time t can be not suitable at the following time.
Using this technique, the image obtained risks to become a puzzle of portions of tissue exposed in very different way during the same acquisition, making comparative analyses very difficult between tissues having similar features, but positioned in points far away from each other, as e.g. in the case of bone density between right and left mandibular condyle.
A third way of solving the present technical problem consists in devices for measuring patient's dimensions of skull and size (e.g. height and skull diameter) based on an alleged correlation between these external measures and bone tissues thickness and density. One method of this kind is described in patent EP 1161122 (Palodex). Such correlation is purely statistical and does not take into any account the anatomical, pathological or personal peculiarities of the patient (e.g. osteoporosis, bone age, etc.).
According to a further aspect, the present invention relates to dental radiographic apparatuses, known as panoramic apparatuses and in particular to a method and an apparatus capable of performing a scout acquisition, preceding the acquisition of a real panoramic image, in a particularly efficient way.
This additional aspect can be independent or combined with the above illustrated additional aspects.
Typically, a panoramic apparatus comprises an X-ray source and detector fixed to the ends of a rigid support, while the patient is positioned in an intermediate position between the X-ray source and detector.
In the known art, apparatuses are known wherein the X-ray source and detector are moved according to pre-set trajectories due to the combination of three movements, i.e. a rotational movement around an axis of rotation of the rigid support due to which X-ray source and detector are moved along a circular trajectory around said axis and around the patient, and a translational movement of the axis of rotation of support arm according at least one or two different directions in the horizontal plane perpendicular to the axis of rotation of the support arm. Moreover, apparatuses are also known, in which the displacement according to two directions in the plane perpendicular to the axis of rotation of the support arm occurs not in a translational way but in a rotational way, due to an angular transfer of the axis of rotation of the support arm around an axis of rotation parallel to the axis of rotation of the support arm, as described e.g. in WO 2011064449A1 (Planmeca).
A panoramic apparatus also comprises a patient positioning and immobilizing device for holding the patient during the acquisition.
In the known art, in the normal working flow a human operator must perform some manual operations for setting the apparatus in order to achieve an image of good quality. Such operations comprise patient positioning and the choice of technical exposure factors, up to the setting of additional options provided by the apparatus, in order to obtain a trajectory of X-ray source and detector of the panoramic apparatus during the following acquisition as accurate as possible of the single patient's Welander curve.
Nowadays, the majority of manual operations are left to the operator's experience and competence. Many apparatuses have devices helping the operator in performing manual operations: e.g. the apparatus proposes to the operator pre-defined settings of technical exposure factors, suitably adjusted according to the patient's gender and size. Such a system is described e.g. in U.S. Pat. No. 4,618,974 (Siemens). However, the selection of those settings is always left to a human operator. The system described in U.S. Pat. No. 4,618,974 is based on statistical criteria and on operator competence.
Technical exposure factors (X-ray tube power (kV), X-ray tube current (mA), duration of exposure (sec) and film speed) and/or acquisition trajectory may be adjusted more precisely by performing a pre-acquisition of the single patient, known in the art as a scout image.
Different kinds of scout image are known in the art. A first kind acquires the entire area of interest, from which the operator then extracts a Region Of Interest (ROI). Based on ROI, technical exposure factors are then set. A second kind consists in acquiring a scout having reduced dimensions with respect to the area of interest, assuming that this small area is representative of the total area of interest, as described e.g. in U.S. Pat. No. 7,519,155 (Morita).
Another way in the known art consists in acquiring a patient's image in the visible range, to then adjust panoramic trajectory to the anatomy of that specific patient (face scan). A patent describing the use of a camera connected to a panoramic apparatus is e.g. U.S. Pat. No. 6,081,739 (Lemchen).
The evolution of the known art is going toward making the adjustment of technical exposure factors and/or acquisition trajectory more and more automatic and operator-independent. Therefore, methods and devices automatically obtaining the necessary information from the scout image are being designed.
For the success of automatisms and for better patient comfort, the time interval between scout acquisition and panoramic real acquisition should be as short as possible, in order to prevent patient movement between scout and real panoramic acquisition.
Two different problems may occur due patient movement:
In a first case, the patient moves during real panoramic acquisition, producing motion artifacts.
In a second case, the patient stands still during scout acquisition and real panoramic acquisition, but moves between scout acquisition and real panoramic acquisition, invalidating the assumption that the scout is representative of the area to be acquired during the real panoramic acquisition.
In the second case, an operator needs to obtain a scout of a broader area, administering a higher X-ray dose to the patient. The operator must always to keep into account that X-rays can lead to patient biological damage, as X-rays are ionizing radiations which can damage cell DNA. Therefore, the need is apparent to administer the lowest possible X-ray dose to a patient under examination.